Add a containerized dep target

[ijc]

To do so first rename the vendor target in docker.Makefile to check-vendor and then add a new vendor target there:

commit 7a03644f839230c6f22fd4caa6d30ad254cbddb7
Author: Ian Campbell <ijc@docker.com>
Date:   Wed Mar 13 13:31:35 2019 +0000

    Add a containerized make target to revendor

    Uses the exact same version as the CI.

    This avoids needing to rely on everyone having the same version of `dep`
    installed in order to avoid spurious changes (especially inconvenient if you
    have multiple projects with different ideas).

    It's slightly annoying because (on Linux at least) it causes `vendor/*` to
    become owned by root (needing a `sudo chown` to repair), that's less annoying
    than having spurious vendoring changes due to tooling differences. Those who
    disagree can continue to use the non-containerized vendor target.

    Signed-off-by: Ian Campbell <ijc@docker.com>

commit 6c881fcd5f614f31b82f4a1a8a57c85057a5c265
Author: Ian Campbell <ijc@docker.com>
Date:   Wed Mar 13 13:09:17 2019 +0000

    Rename containerized `vendor` make target to `check-vendor`

    Having `make vendor` and `make -f dockerfile.Makefile vendor` do different
    things (the first updates, the second checks) is confusing.

    I want to add a containerized vendor update target, which would only be more
    confusing if it weren't called `vendor`.

    Signed-off-by: Ian Campbell <ijc@docker.com>

[codecov[bot]]

Codecov Report

Merging #480 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
+ Coverage   69.41%   69.45%   +0.03%     
==========================================
  Files          50       50              
  Lines        2501     2501              
==========================================
+ Hits         1736     1737       +1     
+ Misses        537      536       -1     
  Partials      228      228

Impacted Files	Coverage Δ
types/parameters/parameters.go	93.65% <0%> (+1.58%)	:arrow_up:

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[silvin-lubecki]

LGTM !